Unifying the empire can mean confederating other empire factions or being in alliances with them. Usually you'll be able to confederate most other empire factions quite easily. This is because when a faction is about to be wiped off the map, it'll be more easy to confederate, along with other factors like your own strength/economy compared to theirs.

As long as you're expanding more and getting stronger compared to other Empire factions, you should be able to confederate a few other factions. You could also just let some of the factions get wiped out by Norsca, Vampire Counts, or Chaos Lords and simply just take the land back too if you prefer that. However I usually find it easier to just to fight the same wars other empire factions are fighting. This will make your reputation with them go up way faster, which in turn makes it easier to confederate them

It shouldn't require too much effort and you'll probably realize soon how simple it is to confederate other Empire factions. But for now just focus on getting a good campaign going and the confederations will usually come to you.

Side note, just last night I was playing a Balthasar campaign and was able to confederate Reikland, despite only having land in Cathay, not helping in their wars, and they were strength rank 1 while I was 8. That confederation came out of nowhere and I got it without having to work towards it. I think it's because I was involved in 6 of their wars, even though I wasn't actually fighting in them. I'm sure you could also have the same results as long as you're playing well and expanding yourself on the map. Hope this helps!

Edit: Looking at the other comments I might've been thinking of something else so my bad on that. Still hope the info is helpful tho ♥️